Question: Is it possible to set a jumper on the Rambo board to prevent someone from being able to overwrite firmware? I'd like a user to have to remove the jumper to update firmware.
Reply: In theory you could use any free IO pin. You connect that pin to ground. Eg an unused endstop pin. Then you need to modify the boot loader to setup that IO pin, enable pullups and check if it is high or low, if low quit the boot loader.
Nothing can stop someone with a avrisp... and you will need a avrisp to upload a new bootloader to each board.
This is just a boot loader firmware issue, not specific to Rambo.